A photodiode generates a current signal corresponding to a received optical signal. The current signal is converted into a voltage signal by a transimpedance circuit, and thereafter, a pulse signal is generated. In general, the larger the amount of light the photodiode receives, the larger the electric current signal the photodiode produces.
However, in an initial-stage transistor circuit mounted in the inside of the transimpedance circuit, when a current signal produced from a photodiode is increased, the initial-stage transistor circuit may become saturated. When the initial-stage transistor circuit is saturated, distortion is generated in the voltage signal and hence, a finally output pulse signal is also distorted.